1. Field of the Invention
The present invention relates to a method of fabricating a polycrystalline silicon thin film, and more particularly, to a method of fabricating a polycrystalline silicon thin film for improving crystallization characteristics and a method of fabricating a liquid crystal display device using the same.
2. Description of the Related Art
Recently, because of the need for an information display and a high demand for using a portable information systems, light and thin film type flat panel display (FPD) devices have been actively being researched and commercialized such that the conventional device of a cathode ray tube (CRT) is being replaced. Among these flat panel display devices, a liquid crystal display (LCD) device is used for displaying an image by using an optical anisotropy of a liquid crystal. A liquid crystal display can be used in notebook computer, a desktop monitor, and other display devices due to its excellent resolution, color rendering capability and picture quality.
An active matrix (AM), a typical driving method used in the liquid crystal display device, drives the pixels of a pixel region in a liquid crystal display by using an amorphous silicon thin film transistor (a-Si TFT) as a switching device in each of the pixels. The amorphous silicon thin film transistor technique was described by English LeComber et al. in 1979, and commercialized as a 3-inch liquid crystal portable television in 1986. Recently, a thin film transistor liquid crystal display having a display area of more than 50 inches has been developed. However, the field effect mobility of the amorphous silicon thin film transistor of about 1 cm2/Vsec prevents its use in peripheral circuits that apply signals to the pixel region, since peripheral circuits operate at more than 1 MHz. Accordingly, research for simultaneously forming a switching transistor in a pixel region and peripheral circuits in a driving circuit region together on a glass substrate by using polycrystalline silicon (poly-Si) thin film transistor having a field effect mobility greater than that of the amorphous silicon thin film transistor has been actively pursued.
The polycrystalline silicon thin film transistor has been applied to a small flat panel displays, such as the eyepiece of a camcorder, since a liquid crystal color television was developed in 1982. Such a thin film transistor has a low photosensitivity and a high field effect mobility as well as can be directly fabricated on a substrate to form driving circuits. Increased mobility can increase an operation frequency of the driving circuits. The frequency capability of the driving circuits determines the number of pixels that can be driven while maintaining an adequate display capability. More specifically, the increased frequency decrease the charging time of a signal applied to a pixel such that distortion of the signal is decreased and picture quality is increased. Compared to the amorphous silicon thin film transistor, which has a high driving voltage of about 25V, the polycrystalline silicon thin film transistor, which has a driving voltage of under 10V, consumes less power.
The polycrystalline silicon thin film transistor can be fabricated by being directly deposited or by depositing an amorphous silicon thin film that is then crystallized with a thermal process. To use a cheap glass substrate, a method for performing low temperature processing and increasing a field effect mobility of a thin film transistor is required. The thermal processing methods for crystallizing the amorphous silicon thin film basically include the solid phase crystallization (SPC) method and the excimer laser annealing (ELA) method.
The solid phase crystallization method forms a polycrystalline silicon thin film at a low temperature of approximately 600° C. In this method, a polycrystalline silicon thin film is formed by depositing an amorphous silicon thin film on a glass substrate having a low melting point and then performing a slow heating process for up to tens of hours at approximately 600° C. A polycrystalline silicon thin film obtained by the solid phase crystallization method has grains of a comparatively large size corresponding to several μm (micrometers). However, there are many defects in the grains. Although not as bad as grain boundaries in a polycrystalline thin film transistor, these defects are a bad influence on a performance of a polycrystalline silicon thin film transistor.
The excimer laser annealing is a typical method of fabricating a polycrystalline silicon thin film transistor at a low temperature. The excimer laser crystallizes an amorphous silicon thin film by radiating a high energy laser beam onto the amorphous silicon thin film for a time of tens of nanoseconds. In this method, the amorphous silicon is melted and crystallized in a very short moment, so that the glass substrate is not damaged at all. A polycrystalline silicon thin film fabricated using the excimer laser also has excellent electric characteristics compared to a polycrystalline silicon thin film fabricated by a general thermal processing method. For example, whereas a field effect mobility of an amorphous silicon thin film transistor is 0.1˜0.2 cm2/Vsec and a field effect mobility of a polycrystalline silicon thin film transistor fabricated by a general thermal processing method is 10˜20 cm2/Vsec, and a field effect mobility of a polycrystalline silicon thin film transistor fabricated using the excimer laser method is more than 100 cm2/Vsec (IEEE Trans. Electron Devices, vol. 36, no. 12, p.2868, 1989).
Hereinafter, a crystallization method using the excimer laser will be explained in more detail. FIG. 1 is a graph showing a grain size of a polycrystalline silicon thin film corresponding to irradiated laser energy density used to form the polycrystalline silicon thin film. As shown in FIG. 1, in the first region A and in the second region B, the more the laser energy density is increased, the grain size of a polycrystalline silicon thin film is increased as discussed in IEEE Electron Device Letters, DEL-7, 276, 1986. However, in the third region C, if energy more than a specific energy density Ec is irradiated, the grain size of a crystallized polycrystalline silicon thin film drastically decreases. That is, according to an irradiated laser energy density graph shown in FIG. 1, the silicon crystallization mechanism of a silicon thin film becomes different past a specific energy density Ec.
FIGS. 2A to 2C are sectional views showing a silicon crystallization mechanism corresponding to the laser energy density graph of FIG. 1. A crystallization mechanism of an amorphous silicon during laser annealing is influenced by many factors, such as laser irradiation conditions including laser energy density, an irradiation pressure, a substrate temperature, and physical/geometrical characteristics including absorption coefficient, thermal conductivity, mass, impurity containing degree and amorphous silicon layer thickness. Amorphous silicon has a very high absorption coefficient near the wavelength of an excimer laser, thereby absorbing energy within a range of 30˜200 ns at the time the amorphous silicon is being irradiated and melted.
In FIG. 2A, the first region A of FIG. 1 is a partial melting region, and an amorphous silicon thin film 12 is crystallized only up to the dotted line and a size of a grain G1 formed at this time corresponds to hundreds of Å. If a laser beam is irradiated on the amorphous silicon thin film 12 on a substrate 10 where a buffer layer 11 is formed, the amorphous silicon thin film 12 is melted. As a strong laser energy is irradiated directly onto a surface of the amorphous silicon thin film 12, a weaker laser energy is irradiated on a lower portion of the amorphous silicon thin film 12 such that crystallization is performed down to a certain part of the amorphous silicon thin film 12. Typically, crystal growth by laser crystallization is performed through a process of melting a surface layer of an amorphous silicon in accordance with the laser irradiation followed by latent heat generation in a lower layer corresponding to a solidification of the surface layer and a melting of a lower layer followed by a solidification of the lower layer. These crystal growth processes will be explained in more detail as follows.
An amorphous silicon thin film on which a laser is irradiated has a melting temperature of more than 1200° C. and primarily melts into a liquid state. Then, since the surface melted layer has a greater temperature difference from a lower silicon and a substrate, the surface melted layer cools fast with a quenching speed of more than 109K/s until a solid phase nucleation and a solidification are achieved. The surface layer is melted until the solid phase nucleation and the solidification are achieved. The melting state lasts for a long time when the laser energy density is high or a thermal emission to the outside is low. Since the surface layer is melted at a lower temperature than a melting temperature of 1400° C. for crystalline silicon, the surface layer is cooled and maintained as a super-cooled state where a temperature is lower than a phase transition. The greater the super-cooling state is, that is, the lower a melting temperature of a thin film or the faster a cooling speed is, the larger the nucleation rate is at the time of solidification such that fine crystal growth results.
When solidification starts as the melted surface layer is cooled, crystal growth proceeds in an upward direction from a crystal nucleus. At this time, latent heat according to a phase transition of the melted surface layer from a liquid state to a solid state is generated and thus secondarily melts a lower amorphous silicon thin film. Then, a solidification of the lower amorphous silicon thin film occurs resulting in crystal growth. At this time, a nucleus generation rate of the lower second melted layer is increased because the lower amorphous silicon thin film is in more of a super cooled state than the first melted layer. Thus, the crystal size resulting from the second melted layer is smaller. To improve crystalline characteristics resulting from crystallization by a laser annealing, the cooling speed of solidification has to be reduced. Cooling speed can be reduced by restraining absorbed laser energy from being emitted out by heating the substrate, double beam irradiation, and/or by a buffer insulating layer between the substrate and the amorphous silicon layer.
FIG. 2B is a sectional view showing a silicon crystallization mechanism of the second region B of FIG. 1, in which the second region B represents a near-completely crystallized region. In FIG. 2B, a polycrystalline silicon thin film having grains G2 of a large size corresponding to 3000˜4000 Å is formed up from an interface of the lower buffer layer 11 and the amorphous silicon thin film 12. That is, according to an energy density corresponding to said region, the amorphous silicon thin film 12 is melted down to a region near the buffer layer 11, so that dense grains on the interface between the amorphous silicon thin film 12 and the buffer layer 11 serve as a nucleus N such that solidification occurs in all directions when crystallization occurs. Accordingly, large grained polycrystalline silicon is formed, as discussed in the Journal of Applied Physics 82, 4086. However, a distribution density of the crystal nucleus N is not uniform. Further, the size of the grains are not uniform. Also, a protuberance portion P is formed on the top surface of the grains G2 that lowers the physical characteristics of the polycrystalline silicon thin film.
FIG. 2C is a sectional view showing a silicon crystallization mechanism of the third region C of FIG. 1 corresponding to a completely crystallized region. In FIG. 2C, grains G3 having a very small size are irregularly formed with the energy density corresponding to said region. That is, when a laser energy density becomes more than a certain level Ec, the amorphous silicon thin film 12 of an irradiated region is all melted and a nucleus which can grow as grains does not exist. Thus, when the amorphous silicon thin film 12 irradiated by a laser of strong energy is drastically cooled, a large number of crystal nucleuses N are generated and minute grains G3 are formed from said crystal nucleuses.
Different from a single crystal, a polycrystalline silicon thin film formed by said crystallization mechanism has a grain boundary. The grain boundary results from a thermal stress generated as melted amorphous silicon is solidified. Grain boundaries lowers a device's electrical characteristics. In order to obtain a high mobility, the density of grain boundaries has to be low, which can be achieved by increasing the grain size of a crystallized polycrystalline silicon thin film. However, in a crystallized polycrystalline silicon thin film in a liquid crystal display, the uniformity of grain size and the morphology of grains are more important than a grain size.
In the case of grains in an active layer constituting a thin film transistor in a liquid crystal display device, if the grain size is not uniform, each thin film transistor will have a different field effect mobility. Thus, there will be a non-uniformity of picture quality across an entire display panel. Also, if a channel of a thin film transistor is not formed in parallel with a longitudinal direction of grains (that is, a direction in which the number of times to meet with a grain boundary is less), a high mobility can not be obtained even if a grain size is large. This is because the grain boundary acts as an obstacle blocking movement of a carrier so as to decrease mobility. This direction-dependent characteristic of grains causes problems, especially in a thin film transistor of a driving circuit region.